Gordian knots of Metrology

The expression «cut the Gordian knot» means to find a non-trivial, bold and simple solution to a complex, confusing issue. It is unlikely that someone could say how many Gordian knots arose in practical metrology, especially in the development of national standards of units and scales of measurement. Each «know-how» — proposed and implemented – is another Gordian knot untied. The author tells about these «knots» in this article.